3 Marks Question

Question 13 Marks

Explain optical isomerism in co$-$ordination compound.

Answer

$\rightarrow$ Optical isomers are mirror images that cannot be superimposed on one another.
$\rightarrow$ These are called as enantiomers.
$\rightarrow$ The molecules or ions that cannot be superimposed are called chiral.
$\rightarrow$ The two forms are called dextro $(d) $ and laevo $(1)$ depending upon the direction they rotate the plane of polarised light in a polarimeter
$\rightarrow$ Dextro $(d)$ rotates plane polarised light to the right, and leaevo $(1)$ rotates plane polarised light to the left.

$\rightarrow$ Optical isomerism is common in octahedral complexes involving didentate ligands
$\rightarrow$ In a coordination entity of the type $\ce{[PtCl_2(en)_2]^{2+}},$ only the $cis$ isomer shows optical activity.

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Question 23 Marks

Write down rules for $\text{IUPAC}$ naming of Co ordination entities.

Answer

$\rightarrow$ The following rules are used when naming coordination compounds:
$(i)$ The cation is named first in both positively and negatively charged coordination entities.
$(ii)$ The ligands are named in an alphabetical order before the name of the central atom / ion.
$(i) $ Names of the anionic ligands end in $-o,$ whereas those of neutral and cationic ligands are the same except aqua for $\ce{H_2O},$ ammine for $\ce{NH_3},$ carbonyl for $\ce{CO}$ and nitrosyl for $\ce{NO}$.
While writing the formula of coordination entity, these are encloded in brackets .
$(iv)$ Prefixes mono, di, tri, etc., are used to indicate the number of the individual ligands in the coordination entity. When the names of the ligands include a numerical prefix, then the terms, bis, tris, tetrakis are used, the ligand to which they refer being placed in parentheses.
For example $,\ce{ [NiCl_2(PPh_3)_2]}$ is named as dichloridobis $($triphenylphosphine$)$ nickel $(II).$
$(v)$ Oxidation state of the metal in cation, anion or neutral coordination entity is indicated by roman numeral in parenthesis.
$(vi)$ If the complex ion is a cation, then the metal is named same as the element.
For example, $Co$ in a complex cation is called cobalt and $Pt$ is called platinum. If the complex ion is an anion, the name of the metal ends with the suffix $-$ ate.
For example $, Co$ in a complex anion $,\ce{ [Co(SCN)_4]^{2-}}$ is called cobaltate.
For some metals, the Latin names are used in the complex anions. e.g. ferrate for $Fe$.
$(vii)$ The neutral complex molecule is named similar to that of the complex cation.

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Question 33 Marks

Write down rules to write the formulas of Mononuclear coordination entities.

Answer

(i) The central atom is listed first.
(ii) The ligands are then listed in alphabetical order. The placement of a ligand in the list does not depend on its charge.
(iii) Polydentate ligands are also listed alphabetically. In case of abbreviated ligand, the first letter of the abbreviation is used to determine the position of the ligand in the alphabetical order.
(iv) The formula for the entire coordination entity, whether charged or not, is enclosed in square brackets. When ligands are polyatomic, their formulas are enclosed in parentheses. Ligand abbreviations are also enclosed in parentheses.
(v) There should be no space between the ligands and the metal within a coordination sphere.
(vi) When the formula of a charged coordinationentity is to be written without that of the counterion, the charge is indicated outside the square brackets as a right superscript with the number before the sign. For example, $\left[ Co ( CN )_6\right]^{3-},[ Cr$ $\left.\left( H _2 O \right)_6\right]^{3+}$, etc.
(vii) The charge of the cations (s) is balanced by the charge of the anions (s).

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Question 43 Marks

Define the following terms: (1) Coordination entity (2) Central atom / ion.

Answer

(1) Coordination entity:
→ "A coordination entity constitutes a central metal atom or ion bonded to a fixed number of ions or molecules."
→ For example, $\left[ CoCl _3\left( NH _3\right)_3\right]$ is a coordination entity in which, the cobalt ion is surrounded by three ammonia molecules and three chloride ions.
→ Other examples are $\left[ Ni ( CO )_4\right],\left[ PtCl _2\left( NH _3\right)_2\right]$, $\left[ Fe ( CN )_6\right]^{4-}$ and $\left[ Co \left( NH _3\right)_6\right]^{3+}$
(2) Central atom/ion:
→ "In a coordination entity, the atom/ion to which a fixed number of ions/groups are bound in a definite geometrical arrangement around it, is called the central atom or ion."
→ For example, the central atom/ion in the coordination entities:

Co-ordination entities	Central atom / ion
$\left[ NiCl _2\left( H _2 O \right)_4\right]$	$Ni ^{2+}$
$\left[ CoCl \left( NH _3\right)_5\right]^{2+}$	$Co ^{3+}$
$\left[ Fe ( CN )_6\right]^{3-}$	$Fe ^{3+}$

→ These central atoms/ions are also referred to as Lewis acids.

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Question 53 Marks

Explain Crystal field splitting in tetrahedral complax.

Answer

→ In tetrahedral coordination entity formation, the d orbital splitting is inverted and is smaller as compared to the octahedral field splitting.
→ For the same metal, the same ligands and metal-ligand distances, it can be shown that $\Delta_t=(4 / 9) \Delta_0$.
→ The orbital splitting energies are not sufficiently large for forcing pairing and, therefore, low spin configurations are rarely observed.

d orbital splitting in a tetrahedral crystal field.
→ The 'g' subscript is used for the octahedral and square planar complexes which have centre of symmetry.
→ Since tetrahedral complexes lack symmetry, 'g' subscrint is not used with energy levels
→ The energy of two e orbitals will decrease by $\frac{3}{5} \Delta t$ while that of three $t_2$ orbitals will increase by $\frac{2}{5} \Delta t$.

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